Fuel ethanol is currently produced from feedstocks such as cornstarch, sugar cane, and sugar beets. However, the production of ethanol from these sources cannot expand much further due to limited farmland suitable for the production of such crops and competing interests with the human and animal food chain. Finally, the use of fossil fuels, with the associated release of carbon dioxide and other products in the conversion process, is a negative environmental impact of the use of these feedstocks
The possibility of producing fuel ethanol from cellulose-containing feedstocks, such as agricultural wastes, grasses, forestry wastes, and sugar processing residues has received much attention due to the availability of large amounts of these inexpensive feedstocks, the desirability to avoid burning or landfilling cellulosic waste materials, and the cleanliness of ethanol as a fuel compared to gasoline. In addition, a byproduct of the cellulose conversion process, lignin, can be used as a fuel to power the cellulose conversion process, thereby avoiding the use of fossil fuels. Studies have shown that, taking the entire cycle into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The cellulosic feedstocks that may be used for ethanol production include (1) agricultural wastes such as corn stover, wheat straw, barley straw, canola straw, and soybean stover; (2) grasses such as switch grass, miscanthus, cord grass, and reed canary grass; (3) forestry wastes such as aspen wood and sawdust; and (4) sugar processing residues such as bagasse and beet pulp.
Cellulose consists of a crystalline structure that is very resistant to breakdown, as is hemicellulose, the second most prevalent component. The conversion of cellulosic fibers to ethanol requires: (1) liberating cellulose and hemicellulose from lignin or increasing the accessibility of cellulose and hemicellulose within the cellulosic feedstock to cellulase enzymes; (2) depolymerizing hemicellulose and cellulose carbohydrate polymers to free sugars; and (3) fermenting the mixed hexose and pentose sugars to ethanol.
The feedstock is conveyed into the plant and the feedstock particles are typically reduced to the desired size to be suitable for handling in the subsequent processing steps. The next process step is a chemical treatment, which generally involves the use of steam or heated water along with acid or alkali to break down the fibrous material. The chemical treatment is carried out either as a direct conversion process—acid hydrolysis or alkali hydrolysis—or as a pretreatment prior to enzymatic hydrolysis.
In the acid or alkali hydrolysis process, the feedstock is subjected to steam and acid or alkali under conditions sufficient to hydrolyze the cellulose to glucose (Grethlein, J. Appl. Chem. Biotechnol., 1978, 28:296-308). The glucose is then fermented to ethanol using yeast, and the ethanol is recovered and purified by distillation.
The enzymatic hydrolysis process involves pretreating the cellulosic material in a process that is analogous to the acid or alkali hydrolysis process described above, but using milder conditions. This pretreatment process increases the accessibility of cellulose within the cellulosic fibers for subsequent conversion steps, but results in little conversion itself. In the next step, the pretreated feedstock is adjusted to an appropriate temperature and pH for enzymatic conversion of cellulose by cellulase enzymes. The reaction conditions for the pretreatment process are chosen to be significantly milder than that in the acid or alkali hydrolysis process, such that the exposed cellulose surface area is greatly increased as the fibrous feedstock is converted to a muddy texture.
In the case of acid pretreatment, much of the hemicellulose is hydrolyzed, but there is little conversion of the cellulose to glucose. The cellulose is hydrolyzed to glucose in a subsequent step that uses cellulase enzymes, and the hydrolysis in this case is known as pretreatment. Alkali pretreatment methods may or may not hydrolyze hemicellulose. In either case, the base reacts with acidic groups present on the hemicellulose to open up the surface of the substrate. In addition, it has been reported that concentrated alkali alters the crystal structure of the cellulose so that it is more amenable to hydrolysis. The cellulose is then typically hydrolyzed to glucose in a subsequent step that uses cellulase enzymes, although it is possible to hydrolyze the cellulose, in addition to the hemicellulose, using acid hydrolysis after alkaline pretreatment.
The hydrolysis of the cellulose, whether by acid, alkali or by pretreatment followed by enzyme hydrolysis, may be followed by the fermentation of the sugar to ethanol, which is then recovered by distillation. Other fermentation products that may be produced include butanol and lactic acid.
The efficient conversion of cellulose from cellulosic material into sugars and the subsequent fermentation of sugars to ethanol or other valuable products represent a major challenge to the industry. In particular, a large amount of impurities, including salt, sugar degradation products, organic acids, soluble phenolic compounds, and other compounds are present in the sugar stream after the pretreatment. These compounds result from degradation of the feedstock or, in the case of the salts, from the acids and alkali added in the process. The presence of these impurities is highly inhibitory to the fermentation of the sugar by the yeast. In the absence of an efficient fermentation of the sugar in high yield, the production of ethanol from biomass is not commercially viable. Furthermore, the inability to recover acetic acid and salt from the sugar streams, due to the large amount of impurities present, represents a loss of potential revenue in the process.
The removal of toxic inhibitors, sulfuric acid and sulfate salts, and acetic acid and acetate salts from the sugar streams prior to fermentation has been the subject of a significant amount of research. The processes studied include lime addition, ion exchange, and ion exclusion.
In lime addition, lime (calcium hydroxide), which is insoluble, is added to the sugar stream to precipitate impurities. The limed sugar solution has an alkaline pH and is neutralized with acid, typically phosphoric acid, sulfurous acid, carbonic acid, or a mixture thereof. Optionally, the lime cake is separated from the sugar by filtration. A second option is to filter the lime cake at alkaline pH and carry out a second filtration to remove material that precipitates during the acidification steps. Lime treatment decreases the toxicity of the sugar stream to yeast and other microbes. However, any handling of the lime cake is difficult and costly. In addition, the introduction of calcium into the stream increases the likelihood that calcium scale will deposit on evaporators, distillation columns, and other process equipment. The clean-up and avoidance of scale increases the cost of sugar processing. Furthermore, the introduction of lime makes the recovery of salt and acetic acid more difficult.
In ion exchange, the sugar stream is flowed through columns packed with ion exchange resins. The resins are in a cation exchange or anion exchange form, or a combination of the two. In principle, cation-exchange resins remove cations such as sodium or potassium, while anion-exchange resins remove anions such as sulfate and acetate. For example, ion exchange has been investigated by Nilvebrant et al. (App. Biochem. Biotech., 2001, 91-93:35-49) in which a spruce hydrolyzate was treated to remove fermentation inhibitors, such as phenolic compounds, furan aldehydes and aliphatic acids. The separation was carried out using an anion exchanger, a cation exchanger and a resin without charged groups. The investigators found that treatment at pH 10.0 using an anionic exchanger removed phenolic inhibitors since at this pH most of the phenolic groups were ionized.
In practice, several factors limit the effectiveness of ion exchange treatment to remove inhibitors. First, the multi-component nature of the streams results in an inefficient removal of some species at any single set of conditions. Second, the high ionic load demands very frequent and expensive regeneration of the resin. Finally, not all of the inhibitors are ionic, and ion exchange is ineffective in removing nonionic compounds from sugar.
Ion exclusion uses ion exchange resins, but rather than bind target ions in solution, the charge on the resin matches that of the target ions in the solution, thereby excluding them from the resin. The excluded compounds then elute from the column readily, while uncharged compounds absorb into the resin and elute from the column more slowly. For example, a concentrated solution of sulfuric acid and glucose has hydrogen as the primary cation. A cation-exchange resin in the hydrogen form will exclude the acid, causing it to elute quickly. The glucose, which is uncharged, is not excluded from the resin and absorbs into the resin void, thereby eluting from the column more slowly than the acid.
Ion exclusion for detoxification of sugars from biomass streams has been described by various groups. For example, Wooley et al., (Ind. Eng. Chem. Res., 1998, 37:3699-3709) teaches the removal of acetic acid and sulfuric acid from biomass sugars by pumping a product stream over a bed of cation exchange resin in the hydrogen form. The positive charge on the resin repels the hydrogen ion in the sulfuric acid, thereby causing the sulfuric acid to elute from the column very quickly. The uncharged sugar molecules are absorbed into the void space of the resin and elute from the column more slowly than the sulfuric acid. Fully associated acetic acid (non-ionic) is a smaller molecule than sugar or sulfuric acid and so elutes from the column more slowly than sulfuric acid or sugar. Also described is a Simulated Moving Bed (SMB) system for producing a glucose stream free of sulfuric acid and acetic acid. A shortcoming of Wooley's process is that the glucose recovery is only 92%. The 8% loss of glucose represents a significant cost in the system. The ion exclusion was carried out at a pH of between about 1-2 and, at such low pH values, significant degradation of xylose is likely.
U.S. Pat. Nos. 5,560,827 and 5,628,907 (Hester et al.) disclose a process for separating an ionic component (acid) from a non-ionic component (sugar) using an SMB arrangement, including a plurality of ion exclusion columns arranged in 4 zones. The separations are run at a low pH using a cationic (or cation-exchange) resin in the hydrogen form. The methods of Hester incorporate various arrangements to minimize the dispersion and channeling effects. The sugar/acid solution is loaded onto the column and the acid elutes first while sugar is eluted later using water.
U.S. Pat. No. 5,407,580 (Hester et al.) discloses a process for separating an ionic component (acid) from a non-ionic component (sugar) using a preparative-scale ion exclusion system. The system includes a floating head distribution plate to prevent evolution of a dilution layer caused by the shrinkage of the resin bed. The columns can be operated over a range of process conditions to produce separate and distinct elution profiles for the acid and sugar. Acceptable conditions for carrying out the process are at a sulfuric acid concentration of 1.0 to 20.0 wt % (fed to the top of the column), a feed volume of 1.0 to 5.0 (percent of empty column volume), a flux rate of 0.1 to 2.0 (cm/min) and using a divinylbenzene resin with a percent crosslinking of between 1.0 and 15% (percent divinylbenzene cross-linking).
U.S. Pat. Nos. 5,580,389 and 5,820,687 (Farone et al.) teach a method of producing and separating sugars. The two-step method involves decrystallizing and hydrolyzing biomass using acid, then pressing the hydrolyzate and collecting the liquid, which contains acid and sugars. The liquid is loaded onto a cross-linked strong cation exchange resin run at low pH, where the sugars adsorb to the resin. The resin is purged with gas, pushing the acid out of the resin; the resin is then washed with water, producing a sugar stream.
U.S. Pat. No. 5,968,362 (Russo et al.) discloses a method of separating sugars and acid by ion exclusion chromatography using an anion exchange resin. The sugars elute through the column, and may contain residual acid and heavy metals. The heavy metals can be removed and the acid neutralized using a lime treatment. The acid adsorbs to the resin and is retained; it is eluted from the resin with water.
Nanguneri et al. (Sep. Sci. Tech., 1990, 25(13-15):1829-1842) simulated the separation of sugars from acids using a modified mathematical model and compared the results obtained with experimental data. Separation performances at different process parameters were then analyzed to determine optimal processing conditions. The simulated process would result in an acid-rich stream eluting first, followed by a dilute acid/sugar interface stream and then a sugar-rich stream. Nanguneri et al. performed an economic analysis at the optimal processing conditions and concluded that ion exclusion is highly feasible for the processing of lignocellulosic feedstocks to produce ethanol. However, a drawback of the method of Nanguneri et al. is that the dilute acid/sugar interface stream is costly to separate and recover.
U.S. Pat. No. 6,663,780 (Heikkilä et al.) discloses a method in which product fractions, such as sucrose, betaine and xylose, are separated from molasses that are obtained from a variety of sources, including beet and cane molasses, as well as hydrolyzates produced from biomass. The process involves treating the molasses with sodium carbonate (pH 9) to precipitate calcium followed by removing the resulting precipitate. The filtrate is then subjected to a simulated moving bed (SMB) process which is carried out using at least two SMB systems packed with a strongly acid cation exchange resin. Sucrose is recovered in a first system and betaine is recovered in a second system. The sucrose obtained from the first system may be crystallized and the crystallization run-off applied to the second system. Also described is a process for recovering xylose from sulphite cooking liquor using two systems. Prior to fractionation in the first system, the sulphite cooking liquor, having a pH of 3.5, is filtered and diluted to a concentration of 47% (w/w). The xylose fractions obtained from the first system are crystallized and, after adjustment to pH 3.6 with MgO, the run-off is fed to the second system. In the second system, a sequential SMB is used to separate xylose from the crystallization run-off.
A disadvantage of the separation technique disclosed in U.S. Pat. No. 6,663,780 (Heikkilä et al.) is that the inclusion of two SMB systems is costly and adds to the complexity of the process. In addition, sugars present in a hydrolyzate produced by the processing of lignocellulosic biomass are much more difficult to crystallize than sucrose in a beet process. The initial sucrose purification by crystallization in U.S. Pat. No. 6,663,780 is not successful with glucose in biomass systems.
Various groups have reported the separation of sucrose from molasses obtained from sugar cane using ion exclusion chromatography or ion exchange. For example, U.S. Pat. No. 4,359,430 (Heikkilä et al.) discloses a method of recovering betaine from inverted molasses. The molasses are first diluted with water to a concentration of 35-40% and then applied to a column containing a cation exchange resin. On elution with water, a first non-sugar waste fraction is obtained, followed by a second sugar-containing fraction, and a third fraction containing betaine. The betaine is recovered by evaporation and crystallization. Although high levels of betaine are recovered, the patent does not address the recovery of sucrose from the sugar-containing fraction.
U.S. Pat. No. 6,482,268 (Hyöky et al.) also discloses a method of separating sucrose and betaine from beet molasses by a simulated moving bed (SMB) process. Similar to U.S. Pat. No. 6,663,780, the method of Hyöky et al. involves first precipitating calcium from the beet molasses by adding sodium carbonate and filtering the resulting calcium carbonate by filtration. The beet molasses are next applied to a column packed with a strong cation exchanger resin with a divinylbenzene backbone. A sucrose fraction is eluted first, followed by a betaine fraction, which is then concentrated and further fractionated to yield a second sucrose fraction and a second betaine fraction containing some sucrose. The second sucrose and betaine fractions are combined with the sucrose and betaine fractions obtained from the initial fractionation. Although Hyöky et al. describe the separation of sucrose and betaine from beet molasses, in a biomass conversion process, these components would not be present.
A method of separating sugar from molasses using ion exclusion chromatography is taught in GB 1,483,327 (Munir et al.). The ion exclusion column comprises two types of cation exchange resins used in the salt form to help prevent shrinkage of the column bed. Sugar adsorbs to the column and is eluted using decarbonized water adjusted to a pH of greater than 9.
WO 95/17517 (Chieffalo et al.) discloses a method of processing municipal solid waste to recover reusable materials and to make ethanol. Cellulosic material is shredded and pre-treated with acid and lime to remove heavy metals, then treated with concentrated acid (sulfuric) to produce sugars. The sugars and the acid are separated on a strong acidic cation ion exchange resin.
U.S. Pat. No. 4,101,338 (Rapaport et al.) discloses a method of separating salts and sucrose present in blackstrap molasses obtained from sugar cane by ion exclusion chromatography. Prior to ion exclusion chromatography, the molasses are treated by removing organic non-sugar impurities and colour. Various methods are suggested for removing these impurities, including a preferred method utilizing precipitation with iron salts, such as ferric chloride or ferric sulfate, to form flocs. The insoluble flocs are then removed from the molasses stream and the soluble iron salts are removed by the addition of lime and phosphoric acid or inorganic phosphate salts, which raises the pH to above 7.0. The molasses stream is then applied to the ion exchange column to produce fractions containing sucrose and separated salts. A disadvantage of this process is that, upon addition of ferric ions, the molasses has a pH that is in the range of 2.0 to 3.0. At such a low pH, degradation of xylose could occur. Furthermore, Rapaport et al. do not address the separation of acetic acid from sugars.
Organic non-carbohydrate impurities within a lignocellulosic system cannot be removed by the methods of Rapaport et al. According to the method of Rapaport, the amount of solids precipitated by iron salts or ethanol is modest and no solids are removed by centrifugation. By contrast, the sugar streams produced during the processing of lignocellulosic feedstock have a much higher level of organic non-carbohydrate impurities and inorganic salts. Rapaport et al. do not address the processing of such concentrated streams.
U.S. Pat. No. 6,709,527 (Fechter et al.) discloses a process of treating an impure cane-derived sugar juice to produce white sugar and white strap molasses. The process involves subjecting the sugar juice to microfiltration/ultrafiltration to decrease the levels of impurities. The sugar juice is next subjected to ion exchange with a strong acid cation exchange resin in the hydrogen form and then to ion exchange with an anion exchange resin in the hydroxide form. After ion exchange, the resulting sugar solution is concentrated to produce syrup which is then crystallized to produce impure crystallized sugar and white strap molasses. Although the process results in the removal of impurities from the sucrose solution, it would be subject to the limitations associated with ion exchange chromatography described above.
U.S. Pat. No. 4,631,129 (Heikkilä et al.) teaches a method of purifying sugar from a sulfite pulping spent liquor stream. The process involves two steps, in which, during the first step, the pH of the spent sulfite liquor is adjusted to below 3.5 and the stream is passed through a strongly acidic ion exclusion resin to recover two lignosulfonate-rich raffinate fractions and a product stream containing the sugar and consisting of 7.8%-55% lignosulfonate. In the second step, the product stream is adjusted to pH 5.5-6.5. The product stream is then filtered, and applied to a second ion exclusion column to further purify the sugar by separating it from the large amount of lignosulfonates in this stream. A problem with this process is that the use of two ion exclusion systems is costly and adds to the complexity of the process. Moreover, Heikkilä et al. do not quantify or address the separation of compounds present during the processing of biomass such as inorganic salts, including sulfate salts, and acetic acid and other organic acids.
Bipp et al. (Fresenius J. Anal. Chem., 1997, 357:321-325) describes the analytical determination and quantification of sugar acids and organic acids from whey powder hydrolyzates by ion exclusion chromatography. The elution was carried out with 0.005 M sulfuric acid (pH of 2.3) at a temperature of 45° C. and 0.05 M (pH of 1.30) and a temperature of 10° C. Although the analysis demonstrated that the method was suitable for the determination and quantification of organic acids, including sugar acids and acetic acid, the temperatures required for the separation would not be practical in an industrial application. Furthermore, such low pH values would likely result in the production of degradation products.
There is a need for an economical system for obtaining inorganic salt and acetate salt from processing of a cellulosic biomass. The development of such a system remains a critical requirement for the overall process to convert lignocellulosic feedstocks to glucose and subsequently to ethanol or other products.